I have two crossover ideas for you.
Final Fantasy XIII and Fate/Stay Night Crossover Challenge
Snow Villiers is the reincarnation of Shirou Emiya, but his memories and powers do not awaken until he is turned into a L'cie. Snow is the determined hero who wants to save everybody but mostly Serah, his love. For this reason the Shirou part of Snow should be from the Heaven's Feel route where he throws away his ideals to save Sakura. Lightning is kind of like a mixture of Saber and Rin. She's a stoic hero with awesome sword skills but has doubts all the way about how she failed in her responsibility to protect Serah (or Britain for Saber), like Saber. She is violent toward Snow when she thinks he's being an idiot and her sister is his love interest, like Rin. The original idea for this challenge was that Rule Breaker could remove the L'cie brand without taking the powers with it, and the fate of turning into either a Cie'th or a Crystal vanishes with the L'cie brand's removal. I would suggest that Saber be Snow's eidolon, but their gestalt mode would be difficult. Instead of Saber transforming into some type of mount for Snow to ride, I suggest that they merge into one being like the The Fate/Prototype Saber (male Arthur Pendragon). ProtoSaber's Excalibur Proto is EX ranked as opposed to Saber's A++ ranked Excalibur so its a good upgrade. Excalibur Proto overpowers ProtoGilgamesh's Enki, which is EX ranked and as powerful as Ea. A Libra during Snow and saber's Gestalt battle could reveal that Saber "yields to those who show personal history" (either Shirou's or Saber's weapons) and "yields to those who show Reality Marbles". Maybe an OMAKE could show Snow turning into a Cie'th, but his Cie'th form is Shirou's Made of Blades form from when Archer's arm and Reality Marble overran his body. The funny part is that this Cie'th would succeed at rescuing Serah like Shirou rescued Sakura while his body, mind, and soul were dying. This being Shirou, he might get together with Lightning and Saber as well as Serah. Maybe the Shroud of Martin could seal off the L'cie marks like it does to Archer's arm.
TYPE-MOON/Sailor Moon Crossover
The Sailor Senshi are the incarnations of the Aristoteles (TYPEs). Similarly to how TYPE-VENUS later incarnated as a blonde woman with wings, the TYPEs incarnated into a human form. Unfortunately, this resulted in their memories being temporarily sealed off, but they are slowly coming back (like Silver Millennium memories). This is an AU of Angel Notes where instead of Gaia dying and sending out a cry for revenge against humanity, Gaia is invaded by the Negaverse and calls for help from the other planets. The Youma invading from the Negaverse are an alien existence and no one other than wielders of True Magic are able to affect them. The TYPEs (now Senshi) are also an alien existence so are able to kill the Youma. Gaia and the other planets were worried that the mere presence of the TYPEs would do as much harm as the Youma themselves thanks to the TYPEs alien existence rewriting the reality around them. So the TYPEs were incarnated as embryos into the wombs of specifically chosen women across Tokyo, the center of the Negaverse incursion. The embryonic incarnation of the TYPEs used the natural process of birth to cause the world to regard the TYPEs' presence as natural and vice-versa, so there would be no conflict with their presence in the world. Maybe the Moon Castle could be Millennium Castle Brunestud. I am not sure what to make of Arcueid Brunestud. Maybe she could be the Senshi of Earth or a replacement for Mamoru. The TYPE-MOON wiki calls Arcueid ARCHETYPE-EARTH, so I suppose something Earth related if you want to include her in the story. This story was intended to answer the question of how to make Sailor Moon cooler. My answer was to import it into the TYPE-MOON universe.
Edit: fixed the spacing and added the Shroud of Martin idea
